1. Field of the Invention
The present invention relates to a field effect transistor (FET) for use in a microwave band, etc. and a method of making the same.
2. Description of the Prior Art
It is necessary to make source resistance small, in order to obtain superior high-frequency characteristics with regard to a transistor for high-frequency, for example, a GaAs Schottky gate FET (abridged as a GaAs MESFET, hereafter). For example, the source resistance is designed usually not exceeding 5 .OMEGA. with regard to the GaAs MESFET for X band.
For the making the source resistance small, the source electrode and the gate electrode should be formed in a manner that both electrodes are disposed very closely to each other. Particularly, in the case where an active layer is considerably thin, for example, where an n-type active layer is formed by the ion-implanting method, the source resistance is extremely large when the distance between the source electrode and the gate electrode is large. Therefore, when the GaAs MESFET for the X band is produced to have the active layer formed with the ion-implanting method, the distance between the gate electrode and the source electrode, or a region having high impurity concentration in place of the source excede, should not be exceeding 1 .mu.m.
However, for to a conventional production method, wherein the photolithographic process for forming the source electrode and the photolithographic process for forming the gate electrode are separately used by utilizing different masks, on account of error in mask alignment it is extremely difficult to obtain sizes under 1 .mu.m as distances between the gate electrode and the source electrode or a region having high impurity concentration and serving in place of the source electrode
Therefore, a so-called self-alignment method has been proposed recently. This method involves forming the gate electrode and the region having high impurity concentration and serving in place of the source electrode with one photolithographic process. FIGS. 1(a), 1(b) and 1(c) show one embodiment of such a self-alignment method. As shown in FIG. 1(a), after the gate electrode 13 is formed on an n-type region 12 disposed on a semi-insulating GaAs substrate 11, an n-type impurity is deeply ion-implanted. As shown in FIG. 1(b), a high concentration n-type region 14 is formed as a result of the ion-implantation only at such parts that are not covered by the gate electrode 13, since the gate electrode 13 has a masking function against the ion-implanting. Next as shown in FIG. 1(c), the source electrode 15 and the drain electrode 16 are formed on the high concentration n-type region 14. Therefore, the distance between the gate electrode 13 and the high concentration n-type region 14, to serve in place of the source electrode 15, becomes zero.
Such a conventional method, however, has the following practical disadvantages. That is, when the width of the gate electrode 13 is below 0.5 .mu.m, a leak current I.sub.L flows as indicated by an arrow in FIG. 1(c), between the high concentration n-type region part of the source side and the high concentration n-type region part of the drain side through the semi-insulating GaAs substrate 11, since the high concentration n-type region 14 is thicker than the n-type region 12 and hence the semi-insulating GaAs substrate 11 does not perform complete insulation.
On account of the above-mentioned problems, hitherto it is difficult to produce a GaAs MESFET having superior high frequency characteristics.